JCO CRITICALITY ACCIDENT Masashi Kanamori Nuclear Emergency Assistance & Training Center

1. JCO CRITICALITY ACCIDENT Masashi Kanamori Nuclear Emergency Assistance & Training Center Japan Atomic Energy Agency 11601-13 Nishi-jusanbugyo, Hitachinaka, Ibaraki、JAPAN 311-1206 1

2. CONTENTS ■ Outline of the criticality accident ■ Criticality accident ■ Emergency environmental radiation monitoring ■ Criticality termination work ■ Countermeasures taken by the related organizations ■ Environmental radiation monitoring after the accident ■ Radiation doses ■ Lessons from the accident and re-arrangements of emergency preparedness in Japan 2

3. Outline of the criticality accident 3

4. Outline of the criticality accident ■ Criticality accident occurred on September 30, 1999, at the JCO conversion facility at Tokai-mura. ■ The facility was in the operation for the re-conversion of enriched uranium. ■ The excursion continued fornearly 20 hours ,and the total number of fissions during the criticality accident was estimated to be 2 x 1018. ■ 167 residents were asked to evacuate , and about 310, 000 residents wereasked not to leave their homes. ■ This accident was rated as level 4 on INES scale. 4

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6. Criticality accident 6

7. Criticality accident ■ Criticality accident occurred on September 30, 1999, at 10:35 am, at the JCO conversion facility which was in the operation for the re-conversion of enriched uranium. ■ The three workers had used the powdered uranium (U3O8), which is 18.8％enriched-uranium,and dissolved them in the stainless steel container. Concentration of uranium nitrate solution is about 370gU / l. ■ On September 30, 1999, at 10:35 am,the 7th solution in a stainless steel container was poured into the precipitation tank and the solution exceeded the critical mass limit lead to the criticality accident. ■ The precipitation tank was not geometrically safe for criticality. So criticality accidents would occur if much more than the criticality mass was poured. 7

8. ■The excursion continued for nearly twenty hours ,and the total number of fissions during the criticality accident was estimated to be 2 x 1018. ■ Exposures were mainly from direct radiations, some rare gases and radioactive iodine were released into the environment, but the effect was small. ■The two workers who received doses of 16 ~ 25GyEq and 6 ~ 9GyEq died. Another worker whose dose was estimated to be between 2 ~ 3GyEq is still living. ■ As the tank and the building were not damaged, most of the fission products were confined in the tank. ■167residents within arange of 350 meters from the criticality tank were evacuated. About 310, 000 residents within a 10km range were asked by the governor not to leave their homes. 8

9. Timeseries at the initial stage afterthe criticality accident (30 September,1999) 10:35The 7th solution in a stainless steel container was poured into the precipitation tank and the solution exceeded the critical mass limit, so criticality accident occurred. 10:43JCO notified to fire station of Tokai-mura, saying “This is JCO, 3 workers were down at the conversion facility (“facility”was abbreviated.), pleasetake them to the hospital in ambulancesimmediately.” (In Japanese, the pronunciation of “conversion” is the same as that of “epilepsy”.) 10:46The rescue team(3persons) arrived at JCO, but they did not know the situation of the accident, so they had any protective measures for radioactive materials. 11:15JCO notified to STA, saying “Criticality accident possibly occurred.” 9

10. 11:34JCO notified to Tokai-mura government. 11:40JCO detected 0.84mSv/h(γ) at the site boundary and informed to STA. 11:52Sufferers(3 workers) were moved to the hospital bythe ambulances. 12:15Tokai-mura government set up Accident Response Headquarters. 13:55STA recommended Ibaraki-ken, government that residents near the site remain indoors . 14:30STA set up Accident Response Headquarters. 15:00Central government set up Accident Response Headquarters. 15:00The Mayor of Tokai-murarecommended that residents living within a 350 m of the JCO plant evacuate. 10

11. ３０ｋｍ Criticalityaccident Joban Freeway JCO Co Ltd. National Route 6 JR Tokai Sta. JAEA 1km Tokai-mura 11

12. JCO Tokai works JCOTokai Works National Route 6 12

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14. Precipitation tank 14

15. Precipitation tank 15

16. Situation of work near the tank 6-9 GyEq 2-3 GyEq 16-25 GyEq 16

17. JNC TN8440 2001-018 JCO臨界事故の終息作業について　より引用 17 Sketch of precipitation tank drawn at initial stage of the accident

18. Memorandum based on the hearing results of operator(8:30～10:00p.m.) 18

19. Emergency environmental radiation monitoring 19

20. Radiation dose Radiation dose around the site ● ： neutron 　　○：　γ 20

21. Radiation dose around the site （Sep.30,10：00 ー Oct.1,7：00） 9/30 10/1 1.0 空間線量の時間変動from安全委 0.1 0.01 21

22. Points of monitoring posts 10 km 8 km ● 6 km 4 km ● 2 km ● ● JCO ● ● ● ● 22

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24. γ-ray measurement results (10:45a.m.) The installation place of 60Co irradiation institution 24

25. The 4th γ-ray measurement results The installation place of 60Co irradiation institution 25

26. Dose measurement results The site boundary and circumference surveillance zone ofprocessing plant 26

27. Neutron measurement results (17:05p.m.) 27

28. Neutron measurement results 28

29. Criticality termination work 29

30. Stages of criticality termination work The termination work was performed in three stages: [1] Polaroid photography and preparation, [2] water drainage, [3] addition of boron solution. 30

31. Policy of dose control in criticality termination work ■The basic concept was prepared based on the ICRP recommendations and Japanese regulations. ■Based on the Japanese regulations, the radiation dose limit for employees is 50mSv and the dose limit for emergency exposure situations is 100mSv. ■ It seemed difficult to manage doses under 50mSv, so doses up to 100mSv were considered acceptable. ■The Nuclear Safety Commission agreed with this policy of doses objectives. 31

33. View of conversion-building (about 11:00p.m.) 33

34. 1 0.1 0.01 0.001 10 12 14 16 18 20 22 0 2 4 6 8 10 12 Time Progress of the accident γ- Dose Equivalent Rate (mSv/h) at γ area monitor in 1st fac. 10/1 9/30 Draining Ar Pursing B Pouring Accident occurred. 34

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36. Draining of the cooling water from the tank 36

37. 37 JNC TN8440 2001-018 Route of boric acid to the precipitation tank

38. Pouring ofboric acid to the precipitation tank 38

39. Piling up sandbags for shielding 39

40. Countermeasures taken by the related organizations 40

41. Timeseries of criticality accident including countermeasures taken by the related organizations (30 September,1999) 10:35Criticality accident occurred. 10:43 JCO notified to fire station of Tokai-mura 11:15 JCO notified to STA. 11:34 JCO notified to Tokai-mura government. 11:40 JCO detected 0.84mSv/h(γ) at the site boundary and informed to STA. 11:52Sufferers(3 workers) were moved to the hospital bythe ambulances. 41

42. 12:15Tokai-mura government set up Accident Response Headquarters. 14:30 STA set up Accident Response Headquarters. 15:00Central government set up Accident Response Headquarters. 15:00The Mayor of Tokai-murarecommended that residents living within a 350 m of the JCO plant evacuate. 17:05Radiation dose (n) of 4.0mSv/h was detected near the site boundary. 20:30Central government set up Local Accident Response Headquarters at JAERI site. 21:00Accident Response Headquarters(Head: Prime Minister) was established. 42

43. 22:30 Ibaraki-ken, government recommended that residents within a 10 km of the site remain indoors . 23:15It was concluded that the cooling water was drained from the jacket surrounding the precipitation tank. ( 1 October,1999) 2:35～6:04The work to drain the cooling water from the jacket of the precipitation tank was carried out. 6:15The removal of the cooling water from the jacket of the precipitation tank was carried out by forcing the Argon gas. 6:30The radiation dose (n) was lowered to the undetectable level. 8:19～8:39Boric acid was poured to the precipitation tank. 43

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45. Countermeasures for residents Evacuation: ■On September 30, at 3:00PM, the Mayor of Tokai-mura recommended that residents living within a 350 m of the JCO plant evacuate. ■The number of persons to evacuate is 161persons(Tokai-mura) and 6persons(Naka-machi). ■Evacuation continued till October 2, at 6:30PM. Sheltering: ■On September 30, at 10:30PM, Ibaraki prefectural government recommended that residents within a 10 km of the site remain indoors . ■The number of persons to remain indoors is about 310,000persons(Tokai-mura, Cities and towns near Tokai-mura) ■Sheltering continued till October 1, at 2:30PM 45

46. Evacuation for residents （Sep.30 ,15：00 -Oct.1,18：30） Evacuation １６１Persons (Tokai-mura) 46

47. Sheltering (Sep.30, 22：30 -Oct.1,14：30） Sheltering 310,000persons 47

48. Environmental radiation monitoring after the accident 48

49. Radiation doses(n,γ）after the accident μSv/h 49

50. Environmental radiation monitoring Neutron, γ-ray(described above) γ-dose rate by released radioactive materials: 0.24 μGy/h (West 8ｋｍ） Detected activated products (AP) and fission products(FP) : Na-24, Mn-56, Sr-91, I-131, I-133, I-135, Cs-138 AP/FP concentration in environmental samples: negligible value - maximum concentration of I-131 at the site boundary: (1.6-44)x10-9 Bq/cm3 << 1x10-5 Bq/cm3 (limit value) - maximum concentration of I-131in Vegetables (excluding root crop and potato) : 0.037 Bq/g <<2 Bq/g (Indices about ingestion restrictions of food and drink prescribedby nuclear safety commission) 50